Natural gas vehicles (NGVs) operate on the same basic principles as other internal combustion-powered vehicles. Fuel, in the form of natural gas, is mixed with air and fed into a cylinder where the mixture is ignited to move a piston up and down. Natural gas can power vehicles currently powered by gasoline and diesel fuels. However, natural gas is a gas at standard temperature and pressure, rather than a liquid, so certain modifications to the vehicles are required, particularly to the engine and fuel receptacle and storage systems.
Most NGVs operate using compressed natural gas (CNG) so as to reduce the space required to store fuel on-board. CNG is typically stored on-board a vehicle under high pressure (3,000-3,600 pounds per square inch) in cylindrical containers that attach to the top, rear, or undercarriage of the vehicle. As CNG is a gas, direct measurement of the fill level of these containers cannot be accomplished as in gasoline powered vehicles, e.g., with a liquid fill level indicator. Rather, the pressure of the CNG in a storage tank reflects the fill level of the tank.
Fueling NGVs occurs at CNG stations, where natural gas is typically supplied from a local gas utility line at low pressure. There are two types of fueling systems typical employed for NGV refueling: fast-fill systems and time-fill (or slow-fill) systems.
Fast-fill systems require an a high-pressure storage tank system and an on-sight compressor to fill the high-pressure storage tank from the low-pressure gas utility line. NGVs are able to be filled from the high-pressure storage tank in about the same amount of time it takes to fuel a comparable gasoline or diesel fueled vehicle. The compressor systems and high-pressure storage tanks required in fast-fill systems, however, add complexity and cost to the fueling station.
An alternative is a time-fill system, which provides CNG to NGVs directly from a compressor. Time-fill systems typically utilize much smaller compressor systems than fast-fill systems, and do not require as much high-pressure storage capacity; typically a small buffer storage tank is sufficient. As such, time-fill systems have reduced complexity and cost relative to their fast-fill system counterparts. One disadvantage of time-fill system is that refueling takes significantly longer than it does on fast-fill system, so much so that NGV fleets refueled with a time-fill system are typically connected to the system and refueled overnight.
This extended refueling time presents certain logistical and monitoring difficulties for NGV fleet operators. Namely, refueling an NGV takes too long to refuel a fleet of vehicles one at a time. To address this, time-fill stations often use a manifold system with multiple refueling connections so that multiple NGVs may be refueled at the same time from the same gas source. Unfortunately, refueling multiple NGVs from the same manifold at the same time does not allow for simple measurement of how much fuel any individual NGV takes on while refueling.